Condensed Consolidated Balance Sheets (Unaudited) - USD ($) $ in Millions	Dec. 31, 2024	Sep. 30, 2024	Dec. 31, 2023
ASSETS
Utility Plant	$ 8,946.3	$ 8,779.1	$ 8,345.0
Less: Accumulated depreciation and amortization	2,570.3	2,535.8	2,467.3
Net Utility Plant	6,376.0	6,243.3	5,877.7
Non-utility Property (net of accumulated depreciation and amortization of $103.9, $96.8 and $76.5 at December 31, 2024, September 30, 2024, and December 31, 2023, respectively)	982.5	955.3	687.1
Other Investments	118.5	115.3	105.5
Total Other Property and Investments	1,101.0	1,070.6	792.6
Current Assets:
Cash and cash equivalents	11.5	4.5	4.8
Accounts receivable:
Utility	379.6	196.3	428.8
Other	172.3	112.5	150.6
Allowance for credit losses	(34.7)	(31.4)	(35.4)
Delayed customer billings	18.4	12.0	22.0
Inventories:
Natural gas	187.0	208.6	220.6
Propane gas	8.6	8.6	8.6
Materials and supplies	47.0	46.7	47.4
Regulatory assets	89.6	115.4	256.4
Prepayments	41.5	47.6	43.3
Other	67.2	50.5	72.8
Total Current Assets	988.0	771.3	1,219.9
Deferred Charges and Other Assets:
Goodwill	1,171.6	1,171.6	1,171.6
Regulatory assets	1,288.4	1,251.8	1,266.9
Other	350.8	352.1	303.0
Total Deferred Charges and Other Assets	2,810.8	2,775.5	2,741.5
Total Assets	11,275.8	10,860.7	10,631.7
Capitalization:
Common stock (par value $1.00 per share; 70.0 million shares authorized; 58.3 million, 57.7 million, and 55.0 million shares issued and outstanding at December 31, 2024, September 30, 2024, and December 31, 2023, respectively)	58.3	57.7	55.0
Paid-in capital	1,933.7	1,902.2	1,727.4
Retained earnings	1,050.5	1,018.7	997.3
Accumulated other comprehensive income (loss)	24.4	12.1	29.1
Total Shareholders' Equity	3,308.9	3,232.7	3,050.8
Temporary equity	8.4	8.6	14.8
Long-term debt (less current portion)	3,697.7	3,704.4	3,247.8
Total Capitalization	7,015.0	6,945.7	6,313.4
Current Liabilities:
Current portion of long-term debt	42.5	42.0	457.0
Notes payable	1,158.0	947.0	1,047.5
Accounts payable	292.3	237.2	293.8
Advance customer billings	59.2	48.4	25.0
Wages and compensation accrued	28.7	51.5	26.1
Customer deposits	31.6	29.9	28.8
Taxes accrued	61.4	105.2	64.9
Regulatory liabilities	51.0	49.5	7.1
Other current liabilities	266.5	193.2	260.3
Total Current Liabilities	1,991.2	1,703.9	2,210.5
Deferred Credits and Other Liabilities:
Deferred income taxes	838.3	808.4	760.6
Pension and postretirement benefit costs	126.6	146.7	135.5
Asset retirement obligations	586.0	579.9	583.6
Regulatory liabilities	577.2	535.5	487.2
Other noncurrent liabilities	141.5	140.6	140.9
Total Deferred Credits and Other Liabilities	2,269.6	2,211.1	2,107.8
Commitments and Contingencies (Note 11)
Total Capitalization and Liabilities	11,275.8	10,860.7	10,631.7
Depositary Shares [Member]
Capitalization:
Preferred stock ($25.00 par value per share; 10.0 million depositary shares authorized, issued and outstanding at December 31, 2024, September 30, 2024, and December 31, 2023)	242.0	242.0	242.0
Spire Missouri [Member]
ASSETS
Utility Plant	5,555.5	5,420.2	5,080.2
Less: Accumulated depreciation and amortization	1,104.5	1,086.0	1,063.7
Net Utility Plant	4,451.0	4,334.2	4,016.5
Total Other Property and Investments	72.9	70.1	67.9
Accounts receivable:
Other	23.8	22.2	22.2
Allowance for credit losses	(27.9)	(24.9)	(28.9)
Delayed customer billings	9.1	5.7	16.8
Inventories:
Natural gas	120.1	129.6	141.8
Propane gas	8.6	8.6	8.6
Materials and supplies	24.3	24.4	24.6
Regulatory assets	56.7	84.0	212.9
Prepayments	23.2	27.2	22.8
Total Current Assets	537.2	432.5	757.4
Deferred Charges and Other Assets:
Goodwill	210.2	210.2	210.2
Regulatory assets	619.7	588.0	630.8
Other	194.3	193.6	146.0
Total Deferred Charges and Other Assets	1,024.2	991.8	987.0
Total Assets	6,085.3	5,828.6	5,828.8
Capitalization:
Paid-in capital and common stock	887.1	854.9	854.9
Retained earnings	1,164.2	1,110.8	1,049.4
Accumulated other comprehensive income (loss)	(1.9)	(2.0)	(2.4)
Total Shareholders' Equity	2,049.4	1,963.7	1,901.9
Long-term debt (less current portion)	1,803.6	1,803.4	1,485.8
Total Capitalization	3,853.0	3,767.1	3,387.7
Current Liabilities:
Current portion of long-term debt	0.0	0.0	300.0
Advance customer billings	42.4	35.5	14.3
Wages and compensation accrued	16.2	24.2	14.4
Customer deposits	6.5	6.1	5.9
Taxes accrued	25.3	60.2	26.4
Regulatory liabilities	17.4	10.2	0.0
Other current liabilities	104.6	50.6	111.1
Total Current Liabilities	908.6	781.7	1,218.4
Deferred Credits and Other Liabilities:
Deferred income taxes	581.7	567.6	545.8
Pension and postretirement benefit costs	91.1	110.0	102.0
Asset retirement obligations	96.6	95.7	112.2
Regulatory liabilities	486.0	443.3	404.5
Other noncurrent liabilities	68.3	63.2	58.2
Total Deferred Credits and Other Liabilities	1,323.7	1,279.8	1,222.7
Commitments and Contingencies (Note 11)
Total Capitalization and Liabilities	6,085.3	5,828.6	5,828.8
Spire Missouri [Member] | Nonrelated Party [Member]
Accounts receivable:
Utility	296.5	152.9	335.5
Current Liabilities:
Accounts payable	107.3	92.0	106.6
Spire Missouri [Member] | Related Party [Member]
Accounts receivable:
Utility	2.8	2.8	1.1
Current Liabilities:
Notes payable	580.9	495.3	633.3
Accounts payable	8.0	7.6	6.4
Spire Alabama Inc [Member]
ASSETS
Utility Plant	2,995.8	2,966.6	2,879.8
Less: Accumulated depreciation and amortization	1,353.5	1,336.6	1,289.1
Net Utility Plant	1,642.3	1,630.0	1,590.7
Total Other Property and Investments	0.1	0.1	0.0
Current Assets:
Cash and cash equivalents	0.0	1.5	0.0
Accounts receivable:
Other	5.6	6.1	5.7
Allowance for credit losses	(6.0)	(5.7)	(5.7)
Delayed customer billings	8.6	5.7	4.6
Inventories:
Natural gas	31.7	37.3	40.7
Materials and supplies	19.0	18.7	18.9
Regulatory assets	17.4	19.2	31.0
Prepayments	8.3	10.7	6.8
Total Current Assets	150.4	129.4	175.8
Deferred Charges and Other Assets:
Regulatory assets	647.3	642.0	612.6
Other	95.0	94.8	84.2
Total Deferred Charges and Other Assets	742.3	736.8	696.8
Total Assets	2,535.1	2,496.3	2,463.3
Capitalization:
Paid-in capital and common stock	279.4	279.4	284.9
Retained earnings	677.1	668.9	639.2
Total Shareholders' Equity	956.5	948.3	924.1
Long-term debt (less current portion)	711.4	711.3	746.0
Total Capitalization	1,667.9	1,659.6	1,670.1
Current Liabilities:
Current portion of long-term debt	35.0	35.0	0.0
Advance customer billings	14.6	10.9	8.8
Wages and compensation accrued	4.4	7.2	3.6
Customer deposits	21.9	20.8	20.0
Taxes accrued	31.2	34.8	32.6
Regulatory liabilities	27.7	33.8	0.0
Other current liabilities	17.4	13.8	18.2
Total Current Liabilities	273.1	249.4	249.9
Deferred Credits and Other Liabilities:
Deferred income taxes	41.0	35.9	13.0
Pension and postretirement benefit costs	26.7	28.0	26.5
Asset retirement obligations	473.7	468.6	455.8
Regulatory liabilities	26.9	28.3	20.7
Other noncurrent liabilities	25.8	26.5	27.3
Total Deferred Credits and Other Liabilities	594.1	587.3	543.3
Total Capitalization and Liabilities	2,535.1	2,496.3	2,463.3
Spire Alabama Inc [Member] | Nonrelated Party [Member]
Accounts receivable:
Utility	65.3	35.5	73.7
Current Liabilities:
Accounts payable	46.7	38.0	33.7
Spire Alabama Inc [Member] | Related Party [Member]
Accounts receivable:
Utility	0.5	0.4	0.1
Current Liabilities:
Notes payable	67.0	48.4	128.5
Accounts payable	$ 7.2	$ 6.7	$ 4.5